Electrically or hydraulically driven lifting gears are often used for lifting and lowering of loads. Typically these include at least one rotating drive that, by way of a rope or other mechanical construction, such for example in the form of a scissor-type drive, accomplishes lifting or lowering of the load.
In order to provide adequate protection of people and goods, the lifting gear may be equipped with a safety brake intended to decelerate and stop the load in the event of a dangerous operating state, as in the event of an impermissibly high lowering speed of the load. Such unintentional movement of the load may for example arise as a result of an unforeseen interruption of the power supply, in the event of failure of a drive element or of an operating brake, or for other reasons.
It is essential that the safety brake, also sometimes referred to as an “interception brake”, be independent of the operating brake. That is to say that the safety brake must also be able to function when the operating brake fails.
The “Safety Brake for an Electric Chain Hoist” disclosed in German Publication DE 101 48 408 C1 provides a practical arrangement in which a drive shaft of a lifting gear (in this case an electric chain hoist) is equipped with a brake ring, i.e. a friction-type deceleration mechanism. The brake ring is connected to a brake-locking gear disk which, in a safety-related deceleration, latches a locking pawl. A centrifugal-force assembly is connected to the drive to provide activation of the locking pawl. In the event of a failure of the operating brake or of an otherwise excessive lowering speed of the lifting gear, the locking pawl is caused to mesh with the brake-locking gear disk, and the lifting gear or the load of the lifting gear is thus decelerated and stopped.
This prior art arrangement exhibits the disadvantage that because triggering of the safety brake is controlled by centrifugal force, a substantial speed differential between the operationally normal lowering of the load and the actual, out-of-bounds lowering speed is required to positively distinguish between normal lowering of the load and a malfunction. A further disadvantage lies in that testing of the centrifugal-force triggering arrangement can only be performed when loading or experimental lowering of the lifting gear is carried out at very high speed. Testing of the centrifugal-force arrangement and thus of the safety brake is thus associated with high load peaks and corresponding stress on the mechanism. Moreover, current safety rules that apply to particularly critical types of such applications raise issues potentially arising in the certification of prior art safety brakes.